1. Field of the Disclosure
The present disclosure relates to radio broadcast receivers and, in particular, to methods and systems for frequency offset correction in a digital radio broadcast receiver.
2. Background Information
Digital radio broadcasting technology delivers digital audio and data services to mobile, portable, and fixed receivers. One type of digital radio broadcasting, referred to as in-band on-channel (IBOC) digital radio broadcasting, uses terrestrial transmitters in the existing Medium Frequency (MF) and Very High Frequency (VHF) radio bands. HD Radio™ technology, developed by iBiquity Digital Corporation, is one example of an IBOC implementation for digital radio broadcasting and reception.
IBOC digital radio broadcasting signals can be transmitted in a hybrid format including an analog modulated carrier in combination with a plurality of digitally modulated carriers or in an all-digital format wherein the analog modulated carrier is not used. Using the hybrid mode, broadcasters may continue to transmit analog AM and FM simultaneously with higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog-to-digital radio while maintaining their current frequency allocations.
One feature of digital transmission systems is the inherent ability to simultaneously transmit both digitized audio and data. Thus the technology also allows for wireless data services from AM and FM radio stations. The broadcast signals can include metadata, such as the artist, song title, or station call letters. Special messages about events, traffic, and weather can also be included. For example, traffic information, weather forecasts, news, and sports scores can all be scrolled across a radio receiver's display while the user listens to a radio station.
IBOC technology can provide digital quality audio, superior to existing analog broadcasting formats. Because each IBOC signal is transmitted within the spectral mask of an existing AM or FM channel allocation, it requires no new spectral allocations. IBOC digital radio broadcasting promotes economy of spectrum while enabling broadcasters to supply digital quality audio to the present base of listeners.
Multicasting, the ability to deliver several audio programs or streams over one channel in the AM or FM spectrum, enables stations to broadcast multiple streams on separate supplemental or sub-channels of the main frequency. For example, multiple streams of data can include alternative music formats, local traffic, weather, news, and sports. The supplemental channels can be accessed in the same manner as the traditional station frequency using tuning or seeking functions. For example, if the analog modulated signal is centered at 94.1 MHz, the same broadcast in IBOC digital radio broadcasting can include supplemental channels 94.1-1, 94.1-2, and 94.1-3. Highly specialized programming on supplemental channels can be delivered to tightly targeted audiences, creating more opportunities for advertisers to integrate their brand with program content. As used herein, multicasting includes the transmission of one or more programs in a single digital radio broadcasting channel or on a single digital radio broadcasting signal. Multicast content over IBOC digital radio broadcasting transmissions can include a main program service (MPS), supplemental program services (SPS), program service data (PSD), and/or other broadcast data.
The National Radio Systems Committee, a standard-setting organization sponsored by the National Association of Broadcasters and the Consumer Electronics Association, adopted an IBOC standard, designated NRSC-5A, in September 2005. NRSC-5A, the disclosure of which is incorporated herein by reference, sets forth the requirements for broadcasting digital audio and ancillary data over AM and FM broadcast channels. The standard and its reference documents contain detailed explanations of the RF/transmission subsystem and the transport and service multiplex subsystems. Copies of the standard can be obtained from the NRSC at http://www.nrscstandards.org/standards.asp. iBiquity's HD Radio technology is an implementation of the NRSC-5A IBOC standard. Further information regarding HD Radio technology can be found at www.hdradio.com and www.ibiquity.com.
Other types of digital radio broadcasting systems include satellite systems such as Satellite Digital Audio Radio Service (SDARS, e.g., XM Radio™, Sirius®), Digital Audio Radio Service (DARS, e.g., WorldSpace®), and terrestrial systems such as Digital Radio Mondiale (DRM), Eureka 147 (branded as DAB Digital Audio Broadcasting®), DAB Version 2, and FMeXtra®. As used herein, the phrase “digital radio broadcasting” encompasses digital audio and data broadcasting including in-band on-channel broadcasting, as well as other digital terrestrial broadcasting and satellite broadcasting.
In a system configured to broadcast hybrid format signals, i.e. signals including an analog modulated carrier in combination with a plurality of digitally modulated subcarriers, a transmitter typically translates a digital baseband frequency signal to a radio frequency (e.g., an FM carrier frequency in the range of 87.9 MHz to 107.9 MHz). A receiver then receives the radio frequency signal, translates the signal back to the baseband frequency, and demodulates the digital and analog portions of the baseband signal in separate pathways. However, there may be a frequency difference between the transmitter oscillator and the receiver oscillator that causes a frequency error, which can result in the baseband signal at the receiver being offset from DC. As a result, conventional radio receivers may include frequency offset and tracking modules that estimate and remove the initial frequency error and then track additional frequency errors as they occur in the digital signal pathway. However, the present inventors have observed that the initial frequency error may be larger than the receiver can estimate due to limitations of existing frequency offset estimation algorithms, thus increasing the requirement for frequency stability of the receiver oscillator. For example, conventional frequency offset algorithms are typically limited to an offset of +/−14 kHz while the input frequency offset can exceed these limits in some instances.
The inventors of the present disclosure have found that it may be desirable to estimate the initial frequency offset of the digital signal by determining and using the frequency offset of the analog signal. It should be noted that this novel approach is not typically available in pure digital radio receivers, which lack an analog signal from which to obtain a frequency offset.